1. Field of the Invention
This invention relates to an OFDM radio communications system employed in a mobile communications system such as a cellular telephone system.
2. Description of the Related Art
In the OFDM (Orthogonal Frequency Division Multiplexing) radio communications system, a plurality of transmitters use the same carrier frequency for the communications. It is considered to execute, in the transmitters, channel coding repetition of repeating the data symbol over a plurality of subcarriers and spreading code multiplication of spreading using the spread code over a plurality of subcarriers (refer to, for example, R1-060140 Orthogonal Pilot Channel Structure for Sectored Beams in E-UTRA Downlink, NTT DoCoMo, Fujitsu, Mitsubishi Electric, NEC, Sharp, [searched 20 Feb. 2006], Internet <URL: http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1—44/Docs/R1-060140.zip>). If this processing is executed in the transmitter, the receiver is able to receive signals at a high signal-to-noise ratio (SNR).
On the other hand, it is also considered to balance an interference signal component from a transmitter other than a desired transmitter and convert the interference signal component into white noise, in the receiver, in order to execute a stable reception. This is based on the matter that in a case where coding is executed, an error-correcting ability is improved and the receiving performance is enhanced by randomizing an error.
When the receiver executes such a signal reception, the signal is scrambled by a scrambling pattern peculiar to the transmitter or a data signal is interleaved by a pattern peculiar to the transmitter, in the transmitter. At this time, scrambling patterns or interleaving patterns in the transmitters are set to have a completely orthogonal or pseudo-orthogonal relationship with each other.
The interference signal component is made orthogonal or pseudo-orthogonal with a signal transmitted from a desired transmitter, and is thereby canceled or converted into white noise on the receiver side. In addition, since a pilot signal is often dispersed in a frequency direction, pilot orthogonality between the transmitters can also be implemented by frequency hopping of changing the frequency location for every transmitter.
The receiver employed in the above-described system needs to receive the OFDM signal at two stages. At the first stage, the pilot signal is balanced within a frequency-time symbol having a correlation with a channel response value, by taking advantage of the matter that the spreading codes or the scrambling patterns employed for the diffusion of the pilot signal in the transmitters have an orthogonal or pseudo-orthogonal relationship with each other. The interference signal component is thereby converted into white noise, and a channel response from the desired transmitter to the receiver is estimated. Furthermore, the data signal is also processed with the interleaving patterns or scrambling patterns to convert the interference signal component into white noise. At the second stage, channel equalization, i.e. equalization of compensating for the channel distortion which the data signal undergoes is executed by utilizing the channel response obtained at the first stage.
However, the location of the pilot signal in the frame employed in the conventional OFDM radio communications system has a problem that the receiver cannot sufficiently acquire the SN gain of the pilot signal. The pilot signal has a lower density than the data signal. Even if the transmitting power of the pilot signal is set to be greater than the transmitting power of the data signal, the pilot signal energy cannot be sufficiently acquired on the cell boundary. In addition, since the pilot signal is pseudo-orthogonal, the interference component cannot be completely canceled.
For this reason, even if the gain of the data signal is sufficiently acquired by the diffusion or scrambling, accuracy in the channel estimation value is insufficient and accuracy in the data reception is therefore deteriorated.